Internet Engineering Task Force (IETF) D. Hankins
Request for Comments: 7227 Google
BCP: 187 T. Mrugalski
Updates: 3315 M. Siodelski
Category: Best Current Practice ISC
ISSN: 2070-1721 S. Jiang
Huawei Technologies Co., Ltd.
S. Krishnan
Ericsson
May 2014
Guidelines for Creating New DHCPv6 Options
Abstract
This document provides guidance to prospective DHCPv6 option
developers to help them create option formats that are easily
adoptable by existing DHCPv6 software. It also provides guidelines
for expert reviewers to evaluate new registrations. This document
updates RFC 3315.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7227.
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RFC 7227 DHCPv6 Option Guidelines May 2014
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Hankins, et al. Best Current Practice [Page 2]

RFC 7227 DHCPv6 Option Guidelines May 20141. Introduction
Most protocol developers ask themselves if a protocol will work, or
work efficiently. These are important questions, but another less
frequently considered question is whether the proposed protocol
presents itself needless barriers to adoption by deployed software.
DHCPv6 [RFC3315] software implementors are not merely faced with the
task of handling a given option's format on the wire. The option
must fit into every stage of the system's process, starting with the
user interface used to enter the configuration up to the machine
interfaces where configuration is ultimately consumed.
Another frequently overlooked aspect of rapid adoption is whether the
option requires operators to be intimately familiar with the option's
internal format in order to use it. Most DHCPv6 software provides a
facility for handling unknown options at the time of publication.
The handling of such options usually needs to be manually configured
by the operator. But, if doing so requires extensive reading (more
than can be covered in a simple FAQ, for example), it inhibits
adoption.
So, although a given solution would work, and might even be space,
time, or aesthetically optimal, a given option is presented with a
series of ever-worsening challenges to be adopted:
o If it doesn't fit neatly into existing configuration files.
o If it requires source code changes to be adopted and, hence,
upgrades of deployed software.
o If it does not share its deployment fate in a general manner with
other options, standing alone in requiring code changes or
reworking configuration file syntaxes.
o If the option would work well in the particular deployment
environment the proponents currently envision, but it has equally
valid uses in some other environment where the proposed option
format would fail or would produce inconsistent results.
There are many things DHCPv6 option creators can do to avoid the
pitfalls in this list entirely, or failing that, to make software
implementors' lives easier and improve its chances for widespread
adoption.
This document is envisaged as a help for protocol developers that
define new options and for expert reviewers that review submitted
proposals.
Hankins, et al. Best Current Practice [Page 4]

RFC 7227 DHCPv6 Option Guidelines May 20142. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. When to Use DHCPv6
Principally, DHCPv6 carries configuration parameters for its clients.
Any knob, dial, slider, or checkbox on the client system, such as "my
domain name servers", "my hostname", or even "my shutdown
temperature", are candidates for being configured by DHCPv6.
The presence of such a knob isn't enough, because DHCPv6 also
presents the extension of an administrative domain -- the operator of
the network to which the client is currently attached. Someone runs
not only the local switching network infrastructure to which the
client is directly (or wirelessly) attached but the various methods
of accessing the external Internet via local assist services that the
network must also provide (such as domain name servers or routers).
This means that, even if a configuration parameter can be potentially
delivered by DHCPv6, it is necessary to evaluate whether it is
reasonable for this parameter to be under the control of the
administrator of whatever network a client is attached to at any
given time.
Note that the client is not required to configure any of these values
received via DHCPv6 (e.g., due to having these values locally
configured by its own administrator). But, it needs to be noted that
overriding DHCPv6-provided values may cause the client to be denied
certain services in the network to which it has attached. The
possibility of having a higher level of control over client node
configuration is one of the reasons that DHCPv6 is preferred in
enterprise networks.
4. General Principles
The primary guiding principle to follow in order to enhance an
option's adoptability is reuse. The option should be created in such
a way that does not require any new or special case software to
support. If old software that is currently deployed and in the field
can adopt the option through supplied configuration facilities, then
it's fairly certain that new software can formally adopt it easily.
There are at least two classes of DHCPv6 options: simple options,
which are provided explicitly to carry data from one side of the
DHCPv6 exchange to the other (such as name servers, domain names, or
time servers), and a protocol class of options, which require special
Hankins, et al. Best Current Practice [Page 5]

RFC 7227 DHCPv6 Option Guidelines May 2014
processing on the part of the DHCPv6 software or are used during
special processing (such as the Fully Qualified Domain Name (FQDN)
option [RFC4704]), and so forth; these options carry data that is the
result of a routine in some DHCPv6 software.
The guidelines laid out here should be applied in a relaxed manner
for the protocol class of options. Wherever a special case code is
already required to adopt the DHCPv6 option, it is substantially more
reasonable to format the option in a less generic fashion, if there
are measurable benefits to doing so.
5. Reusing Other Option Formats
The easiest approach to manufacturing trivially deployable DHCPv6
options is to assemble the option out of whatever common fragments
fit, possibly allowing a group of data elements to repeat to fill the
remaining space (if present) and thus provide multiple values. Place
all fixed-size values at the start of the option and any variable
-/indeterminate-sized values at the tail end of the option.
This means that implementations will likely be able to reuse code
paths designed to support the other options.
There is a trade-off between the adoptability of previously defined
option formats and the advantages that new or specialized formats can
provide. In general, it is usually preferable to reuse previously
used option formats.
However, it isn't very practical to consider the bulk of DHCPv6
options already allocated and to consider which of those solve a
similar problem. So, the following list of common option format data
elements is provided as shorthand. Please note that it is not
complete in terms of exampling every option format ever devised.
If more complex options are needed, those basic formats mentioned
here may be considered as primitives (or 'fragment types') that can
be used to build more complex formats. It should be noted that it is
often easier to implement two options with trivial formats than one
option with a more complex format. That is not an unconditional
requirement though. In some cases, splitting one complex option into
two or more simple options introduces inter-option dependencies that
should be avoided. In such a case, it is usually better to keep one
complex option.
Hankins, et al. Best Current Practice [Page 6]

RFC 7227 DHCPv6 Option Guidelines May 2014
o NTP Multicast Address [RFC5908] (a single address only)
5.2. Option with Single Flag (Boolean)
Sometimes, it is useful to convey a single flag that can take either
on or off values. Instead of specifying an option with 1 bit of
usable data and 7 bits of padding, it is better to define an option
without any content. It is the presence or absence of the option
that conveys the value. This approach has the additional benefit of
the absent option designating the default; that is, the administrator
has to take explicit actions to deploy the opposite of the default
value.
The absence of the option represents the default value, and the
presence of the option represents the other value, but that does not
necessarily mean that absence is "off" (or "false") and presence is
"on" (or "true"). That is, if it's desired that the default value
for a bistable option is "true"/"on", then the presence of that
option would turn it off (make it false). If the option presence
signifies an off/false state, that should be reflected in the option
name, e.g., OPTION_DISABLE_FOO.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Option for Conveying Boolean
Examples of use:
o DHCPv6 Rapid Commit [RFC3315]
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RFC 7227 DHCPv6 Option Guidelines May 20145.3. Option with IPv6 Prefix
Sometimes, there is a need to convey an IPv6 prefix. The information
to be carried by such an option includes the 128-bit IPv6 prefix
together with a length of this prefix taking values from 0 to 128.
Using the simplest approach, the option could convey this data in two
fixed-length fields: one carrying the prefix length and another
carrying the prefix. However, in many cases, /64 or shorter prefixes
are used. This implies that the large part of the prefix data
carried by the option would have its bits set to 0 and would be
unused. In order to avoid carrying unused data, it is recommended to
store the prefix in the variable-length data field. The appropriate
option format is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| prefix6len | ipv6-prefix |
+-+-+-+-+-+-+-+-+ (variable length) |
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Option with IPv6 Prefix
option-length is set to 1 + length of the IPv6 prefix.
prefix6len is 1 octet long and specifies the length in bits of the
IPv6 prefix. Typically allowed values are 0 to 128.
The ipv6-prefix field is a variable-length field that specifies the
IPv6 prefix. The length is (prefix6len + 7) / 8. This field is
padded with 0 bits up to the nearest octet boundary when prefix6len
is not divisible by 8.
Examples of use:
o Default Mapping Rule [MAP]
For example, the prefix 2001:db8::/60 would be encoded with an
option-length of 9, prefix6-len would be set to 60, and the
ipv6-prefix would be 8 octets and would contain octets 20 01 0d b8 00
00 00 00.
It should be noted that the IAPREFIX option defined by [RFC3633] uses
a full-length 16-octet prefix field. The concern about option length
was not well understood at the time of its publication.
Hankins, et al. Best Current Practice [Page 9]

RFC 7227 DHCPv6 Option Guidelines May 2014
Examples of use:
o Boot File URL [RFC5970]
An alternate encoding to support multiple URIs is available. An
option must be defined to use either the single URI format above or
the multiple URI format below depending on whether a single URI is
always sufficient or if multiple URIs are possible.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. uri-data .
. . . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Option with Multiple URIs
Each instance of the uri-data is formatted as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
| uri-len | URI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
The uri-len is 2 octets long and specifies the length of the URI
data. Although the URI format in theory supports up to 64 KB of
data, in practice, large chunks of data may be problematic. See
Section 15 for details.
5.8. Option with Text String
A text string is a sequence of characters that have no semantics.
The encoding of the text string MUST be specified. Unless otherwise
specified, all text strings in newly defined options are expected to
be Unicode strings that are encoded using UTF-8 [RFC3629] in Net-
Unicode form [RFC5198]. Please note that all strings containing only
7-bit ASCII characters are also valid UTF-8 Net-Unicode strings.
If a data format has semantics other than just being text, it is not
a string; e.g., an FQDN is not a string, and a URI is also not a
string because they have different semantics. A string must not
include any terminator (such as a null byte). The null byte is
treated as any other character and does not have any special meaning.
This option format can be used to carry a text string:
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RFC 7227 DHCPv6 Option Guidelines May 2014
o Location-to-Service Translation (LoST) Server Domain Name
[RFC5223]
o Location Information Server (LIS) Domain Name [RFC5986]
o Dual-Stack Lite (DS-Lite) Address Family Transition Router (AFTR)
Location [RFC6334] (a single FQDN)
o Home Network Identifier [RFC6610] (a single FQDN)
o Home Agent FQDN [RFC6610] (a single FQDN)
6. Avoid Conditional Formatting
Placing an octet at the start of the option that informs the software
how to process the remaining octets of the option may appear simple
to the casual observer. But, the only conditional formatting methods
that are in widespread use today are 'protocol' class options.
Therefore, conditional formatting requires new code to be written and
complicates future interoperability should new conditional formats be
added; existing code has to ignore conditional formats that it does
not support.
7. Avoid Aliasing
Options are said to be aliases of each other if they provide input to
the same configuration parameter. A commonly proposed example is to
configure the location of some new service ("my foo server") using a
binary IP address, a domain name field, and a URL. This kind of
aliasing is undesirable and is not recommended.
In this case, where three different formats are supposed, it more
than triples the work of the software involved, requiring support for
not merely one format but support to produce and digest all three.
Furthermore, code development and testing must cover all possible
combinations of defined formats. Since clients cannot predict what
values the server will provide, they must request all formats. So,
in the case where the server is configured with all formats, DHCPv6
message bandwidth is wasted on option contents that are redundant.
Also, the DHCPv6 option number space is wasted, as three new option
codes are required rather than one.
It also becomes unclear which types of values are mandatory and how
configuring some of the options may influence the others. For
example, if an operator configures the URL only, should the server
synthesize a domain name and an IP address?
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RFC 7227 DHCPv6 Option Guidelines May 2014
A single configuration value on a host is probably presented to the
operator (or other software on the machine) in a single field or
channel. If that channel has a natural format, then any alternative
formats merely make more work for intervening software in providing
conversions.
So, the best advice is to choose the one method that best fulfills
the requirements for simplicity (such as with an IP address and a
port pair), late binding (such as with DNS), or completeness (such as
with a URL).
8. Choosing between an FQDN and an Address
Some parameters may be specified as an FQDN or an address. In most
cases, one or the other should be used. This section discusses pros
and cons of each approach and is intended to help make an informed
decision in that regard. It is strongly discouraged to define both
option types at the same time (see Section 7), unless there is
sufficient motivation to do so.
There is no single recommendation that works for every case. It very
much depends on the nature of the parameter being configured. For
parameters that are network specific or represent certain aspects of
network infrastructure, like available mobility services, in most
cases addresses are a more usable choice. For parameters that can be
considered an application-specific configuration, like SIP servers,
it is usually better to use an FQDN.
Applications are often better suited to deal with FQDN failures than
with address failures. Most operating systems provide a way to retry
an FQDN resolution if the previous attempt fails. That type of error
recovery is supported by a great number of applications. On the
other hand, there is typically no API available for applications to
reconfigure over DHCP to get a new address value if the one received
is no longer appropriate. This problem may be usually addressed by
providing a list of addresses rather than just a single one. That,
on the other hand, requires a defined procedure on how multiple
addresses should be used (all at once, round robin, try first and
fail over to the next if it fails, etc.).
An FQDN provides a higher level of indirection and ambiguity. In
many cases, that may be considered a benefit, but it can be
considered a flaw in others. For example, one operator suggested
that the same name be resolved to different addresses, depending on
the point of attachment of the host doing the resolution. This is
one way to provide localized addressing. However, in order to do
this, it is necessary to violate the DNS convention that a query on a
particular name should always return the same answer (aside from the
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RFC 7227 DHCPv6 Option Guidelines May 2014
ordering of IP addresses in the response, which is supposed to be
varied by the name server). This same locality of reference for
configuration information can be achieved directly using DHCP, since
the DHCP server must know the network topology in order to provide IP
address or prefix configuration.
The other type of ambiguity is related to multiple provisioning
domains (see Section 12). The stub resolver on the DHCP client
cannot at present be assumed to make the DNS query for a DHCP-
supplied FQDN on the same interface on which it received its DHCP
configuration and may, therefore, get a different answer from the DNS
than was intended.
This is particularly a problem when the normal expected use of the
option makes sense with a private DNS zone(s), as might be the case
on an enterprise network. It may also be the case that the client
has an explicit DNS server configured and may, therefore, never query
the enterprise network's internal DNS server.
An FQDN does require a resolution into an actual address. This
implies the question as to when the FQDN resolution should be
conducted. There are a couple of possible answers: a) by the server,
when it is started, b) by the server, when it is about to send an
option, c) by the client, immediately after receiving an option, and
d) by the client, when the content of the option is actually
consumed. For a), b), and possibly c), the option should really
convey an address, not an FQDN. The only real incentive to use an
FQDN is case d). It is the only case that allows possible changes in
the DNS to be picked up by clients.
If the parameter is expected to be used by constrained devices (low
power, battery operated, and low capabilities) or in very lossy
networks, it may be appealing to drop the requirement of performing
the DNS resolution and use addresses. Another example of a
constrained device is a network-booted device, where despite the fact
that the node itself is very capable once it's booted, the boot prom
is quite constrained.
Another aspect that should be considered is time required for the
clients to notice any configuration changes. Consider a case where a
server configures service A using an address and service B using an
FQDN. When an administrator decides to update the configuration, he
or she can update the DHCP server configuration to change both
services. If the clients do not support reconfigure (which is an
optional feature of RFC 3315 but in some environments, e.g., cable
modems, is mandatory), the configuration will be updated on the
clients after the T1 timer elapses. Depending on the nature of the
change (is it a new server added to a cluster of already operating
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RFC 7227 DHCPv6 Option Guidelines May 2014
servers or a new server that replaces the only available server that
crashed?), this may be an issue. On the other hand, updating service
B may be achieved with a DNS record update. That information may be
cached by caching DNS servers for up to Time to Live (TTL).
Depending on the values of T1 and TTL, one update may be faster than
another. Furthermore, depending on the nature of the change (planned
modification or unexpected failure), T1 or TTL may be lowered before
the change to speed up new configuration adoption.
Simply speaking, protocol designers don't know what the TTL or the T1
time will be, so they can't make assumptions about whether a DHCP
option will be refreshed more quickly based on T1 or TTL.
Addresses have the benefit of being easier to implement and handle by
the DHCP software. An address option is simpler to use, has
validation that is trivial (multiple of 16 constitutes a valid
option), is explicit, and does not allow any ambiguity. It is faster
(does not require extra round-trip time), so it is more efficient,
which can be especially important for energy-restricted devices. It
also does not require that the client implements a DNS resolution.
An FQDN imposes a number of additional failure modes and issues that
should be dealt with:
1. The client must have knowledge about available DNS servers. That
typically means that option DNS_SERVERS [RFC3646] is mandatory.
This should be mentioned in the document that defines the new
option. It is possible that the server will return the FQDN
option but not the DNS server's option. There should be a brief
discussion about it;
2. The DNS may not be reachable;
3. The DNS may be available but may not have appropriate information
(e.g., no AAAA records for the specified FQDN);
4. The address family must be specified (A, AAAA, or any); the
information being configured may require a specific address
family (e.g., IPv6), but there may be a DNS record only of
another type (e.g., A only with an IPv4 address).
5. What should the client do if there are multiple records available
(use only the first one, use all, use one and switch to the
second if the first fails for whatever reason, etc.). This may
be an issue if there is an expectation that the parameter being
configured will need exactly one address;
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RFC 7227 DHCPv6 Option Guidelines May 2014
6. Multihomed devices may be connected to different administrative
domains with each domain providing different information in the
DNS (e.g., an enterprise network exposing private domains). The
client may send DNS queries to a different DNS server; and
7. It should be mentioned if Internationalized Domain Names are
allowed. If they are, DNS option encoding should be specified.
Address options that are used with overly long T1 (renew timer)
values have some characteristics of hard-coded values. That is
strongly discouraged. See [RFC4085] for an in-depth discussion. If
the option may appear in Information-request, its lifetime should be
controlled using the information refresh time option [RFC4242].
One specific case that makes the choice between an address and an
FQDN not obvious is a DNS Security (DNSSEC) bootstrap scenario.
DNSSEC validation imposes a requirement for clock sync (to the
accuracy reasonably required to consider signature inception and
expiry times). This often implies usage of NTP configuration.
However, if NTP is provided as an FQDN, there is no way to validate
its DNSSEC signature. This is a somewhat weak argument though, as
providing an NTP server as an address is also not verifiable using
DNSSEC. If the trustworthiness of the configuration provided by the
DHCP server is in question, DHCPv6 offers mechanisms that allow
server authentication.
9. Encapsulated Options in DHCPv6
Most options are conveyed in a DHCPv6 message directly. Although
there is no codified normative language for such options, they are
often referred to as top-level options. Many options may include
other options. Such inner options are often referred to as
encapsulated or nested options. Those options are sometimes called
sub-options, but this term actually means something else and,
therefore, should never be used to describe encapsulated options. It
is recommended to use the term "encapsulated" as this terminology is
used in [RFC3315]. The difference between encapsulated and sub-
options is that the former uses normal DHCPv6 option numbers, while
the latter uses option number space specific to a given parent
option. It should be noted that, contrary to DHCPv4, there is no
shortage of option numbers; therefore, almost all options share a
common option space. For example, option type 1 meant different
things in DHCPv4, depending if it was located in the top level or
inside of the Relay Agent Information option. There is no such
ambiguity in DHCPv6 (with the exception of [RFC5908], which SHOULD
NOT be used as a template for future DHCP option definitions).
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RFC 7227 DHCPv6 Option Guidelines May 2014
From the implementation perspective, it is easier to implement
encapsulated options rather than sub-options, as the implementors do
not have to deal with separate option spaces and can use the same
buffer parser in several places throughout the code.
Such encapsulation is not limited to one level. There is at least
one defined option that is encapsulated twice: Identity Association
for Prefix Delegation (IA_PD), as defined in Section 9 of [RFC3633],
conveys the Identity Association (IA) Prefix (IAPREFIX), as defined
in Section 10 of [RFC3633]. Such a delegated prefix may contain an
excluded prefix range that is represented by the PD_EXCLUDE option
that is conveyed as encapsulated inside IAPREFIX (PD_EXCLUDE is
defined in [RFC6603]). It seems awkward to refer to such options as
sub-sub-option or doubly encapsulated option; therefore, the
"encapsulated option" term is typically used, regardless of the
nesting level.
When defining a DHCP-based configuration mechanism for a protocol
that requires something more complex than a single option, it may be
tempting to group configuration values using sub-options. That
should preferably be avoided, as it increases complexity of the
parser. It is much easier, faster, and less error prone to parse a
large number of options on a single (top-level) scope than to parse
options on several scopes. The use of sub-options should be avoided
as much as possible, but it is better to use sub-options rather than
conditional formatting.
It should be noted that currently there is no clear way defined for
requesting sub-options. Most known implementations are simply using
the top-level Option Request Option (ORO) for requesting both top-
level and encapsulated options.
10. Additional States Considered Harmful
DHCP is designed for provisioning clients. Less experienced protocol
designers often assume that it is easy to define an option that will
convey a different parameter for each client in a network. Such
problems arose during designs of the Mapping of Address and Port
(MAP) [MAP] and IPv4 Residual Deployment (4rd) [SOLUTION-4rd]. While
it would be easier for provisioned clients to get ready to use per-
client option values, such a requirement puts exceedingly large loads
on the server side. The new extensions may introduce new
implementation complexity and additional database state on the
server. Alternatives should be considered, if possible. As an
example, [MAP] was designed in a way that all clients are provisioned
with the same set of MAP options, and each provisioned client uses
its unique address and delegated prefix to generate client-specific
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RFC 7227 DHCPv6 Option Guidelines May 2014
information. Such a solution does not introduce any additional state
for the server and, therefore, scales better.
It also should be noted that contrary to DHCPv4, DHCPv6 keeps several
timers for renewals. Each IA_NA (addresses) and IA_PD (prefixes)
contains T1 and T2 timers that designate time after which the client
will initiate renewal. Those timers apply only to their associated
IA containers. Refreshing other parameters should be initiated after
a time specified in the information refresh time option (defined in
[RFC4242]), carried in the Reply message, and returned in response to
the Information-request message. Introducing additional timers make
deployment unnecessarily complex and SHOULD be avoided.
11. Configuration Changes Occur at Fixed Times
In general, DHCPv6 clients only refresh configuration data from the
DHCP server when the T1 timer expires. Although there is a
Reconfigure mechanism that allows a DHCP server to request that
clients initiate reconfiguration, support for this mechanism is
optional and cannot be relied upon.
Even when DHCP clients refresh their configuration information, not
all consumers of DHCP-sourced configuration data notice these
changes. For instance, if a server is started using parameters
received in an early DHCP transaction, but does not check for updates
from DHCP, it may well continue to use the same parameter
indefinitely. There are a few operating systems that take care of
reconfiguring services when the client moves to a new network (e.g.,
based on mechanisms like [RFC4436], [RFC4957], or [RFC6059]), but
it's worth bearing in mind that a renew may not always result in the
client taking up new configuration information that it receives.
In light of the above, when designing an option you should take into
consideration the fact that your option may hold stale data that will
only be updated at an arbitrary time in the future.
12. Multiple Provisioning Domains
In some cases, there could be more than one DHCPv6 server on a link,
with each providing a different set of parameters. One notable
example of such a case is a home network with a connection to two
independent ISPs.
The DHCPv6 specification does not provide clear advice on how to
handle multiple provisioning sources. Although [RFC3315] states that
a client that receives more than one Advertise message may respond to
one or more of them, such capability has not been observed in
existing implementations. Existing clients will pick one server and
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RFC 7227 DHCPv6 Option Guidelines May 2014
will continue the configuration process with that server, ignoring
all other servers.
In addition, a node that acts as a DHCPv6 client may be connected to
more than one physical network. In most cases, it will operate a
separate DHCP client state machine on each interface and acquire
different, possibly conflicting, information through each. This
information will not be acquired in any synchronized way.
Existing nodes cannot be assumed to systematically segregate
configuration information on the basis of its source; as a result, it
is quite possible that a node may receive an FQDN on one network
interface but do the DNS resolution on a different network interface,
using different DNS servers. As a consequence, DNS resolution done
by the DHCP server is more likely to behave predictably than DNS
resolution done on a multi-interface or multihomed client.
This is a generic DHCP issue and should not be dealt within each
option separately. This issue is better dealt with using a protocol-
level solution, and fixing this problem should not be attempted on a
per-option basis. Work is ongoing in the IETF to provide a
systematic solution to this problem.
13. Chartering Requirements and Advice for Responsible Area Directors
Adding a simple DHCP option is straightforward and generally
something that any working group (WG) can do, perhaps with some help
from designated DHCP experts. However, when new fragment types need
to be devised, this requires the attention of DHCP experts and should
not be done in a WG that doesn't have a quorum of such experts. This
is true whether the new fragment type has the same structure as an
existing fragment type but with different semantics, or the new
format has a new structure.
Responsible Area Directors for WGs that wish to add a work item to a
WG charter to define a new DHCP option should get clarity from the WG
as to whether the new option will require a new fragment type or new
semantics, or whether it is a simple DHCP option that fits existing
definitions.
If a WG needs a new fragment type, it is preferable to see if another
WG exists whose members already have sufficient expertise to evaluate
the new work. If such a working group is available, the work should
be chartered in that working group instead. If there is no other WG
with DHCP expertise that can define the new fragment type, the
responsible AD should seek help from known DHCP experts within the
IETF to provide advice and frequent early review as the original WG
defines the new fragment type.
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RFC 7227 DHCPv6 Option Guidelines May 2014
In either case, the new option should be defined in a separate
document, and the work should focus on defining a new format that
generalizes well and can be reused, rather than a single-use fragment
type. The WG that needs the new fragment type can define their new
option referencing the new fragment type document, and the work can
generally be done in parallel, avoiding unnecessary delays. Having
the definition in its own document will foster reuse of the new
fragment type.
The responsible AD should work with all relevant WG Chairs and DHCP
experts to ensure that the new fragment type document has in fact
been carefully reviewed by the experts and appears satisfactory.
Responsible Area Directors for WGs that are considering defining
options that actually update DHCP, as opposed to simple options,
should go through a process similar to that described above when
trying to determine where to do the work. Under no circumstances
should a WG be given a charter deliverable to define a new DHCP
option, and then on the basis of that charter item actually make
updates to DHCP.
14. Considerations for Creating New Formats
When defining new options, one specific consideration to evaluate is
whether or not options of a similar format would need to have
multiple or single values encoded (whatever differs from the current
option) and how that might be accomplished in a similar format.
When defining a new option, it is best to synthesize the option
format using fragment types already in use. However, in some cases,
there may be no fragment type that accomplishes the intended purpose.
The matter of size considerations and option order are further
discussed in Sections 15 and 17.
15. Option Size
DHCPv6 [RFC3315] allows for packet sizes up to 64 KB. First, through
its use of link-local addresses, it avoids many of the deployment
problems that plague DHCPv4 and is actually a UDP over the IPv6-based
protocol (compared to DHCPv4, which is mostly UDP over IPv4 but with
layer-2 hacks). Second, RFC 3315 explicitly refers readers to
Section 5 of [RFC2460], which describes an MTU of 1280 octets and a
minimum fragment reassembly of 1500 octets. It's feasible to suggest
that DHCPv6 is capable of having larger options deployed over it, and
at least no common upper limit is yet known to have been encoded by
its implementors. It is not really possible to describe a fixed
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RFC 7227 DHCPv6 Option Guidelines May 2014
limit that cleanly divides workable option sizes from those that are
too big.
It is advantageous to prefer option formats that contain the desired
information in the smallest form factor that satisfies the
requirements. Common sense still applies here. It is better to
split distinct values into separate octets rather than propose overly
complex bit-shifting operations to save several bits (or even an
octet or two) that would be padded to the next octet boundary anyway.
DHCPv6 does allow for multiple instances of a given option, and they
are treated as distinct values following the defined format; however,
this feature is generally preferred to be restricted to protocol
class features (such as the IA_* series of options). In such cases,
it is better to define an option as an array if it is possible. It
is recommended to clarify (with normative language) whether a given
DHCPv6 option may appear once or multiple times. The default
assumption is only once.
In general, if a lot of data needs to be configured (for example,
some option lengths are quite large), DHCPv6 may not be the best
choice to deliver such configuration information and SHOULD simply be
used to deliver a URI that specifies where to obtain the actual
configuration information.
16. Singleton Options
Although [RFC3315] states that each option type MAY appear more than
once, the original idea was that multiple instances are reserved for
stateful options, like IA_NA or IA_PD. For most other options, it is
usually expected that they will appear once at most. Such options
are called singleton options. Sadly, RFCs have often failed to
clearly specify whether or not a given option can appear more than
once.
Documents that define new options SHOULD state whether or not these
options are singletons. Unless otherwise specified, newly defined
options are considered to be singletons. If multiple instances are
allowed, the document MUST explain how to use them. Care should be
taken not to assume that they will be processed in the order they
appear in the message. See Section 17 for more details.
When deciding whether single or multiple option instances are allowed
in a message, take into consideration how the content of the option
will be used. Depending on the service being configured, it may or
may not make sense to have multiple values configured. If multiple
values make sense, it is better to explicitly allow that by using an
option format that allows multiple values within one option instance.
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RFC 7227 DHCPv6 Option Guidelines May 2014
Allowing multiple option instances often leads to confusion.
Consider the following example. Basic DS-Lite architecture assumes
that the B4 element (DHCPv6 client) will receive the AFTR option and
establish a single tunnel to the configured tunnel termination point
(AFTR). During the standardization process of [RFC6334], there was a
discussion whether multiple instances of the DS-Lite tunnel option
should be allowed. This created an unfounded expectation that the
clients receiving multiple instances of the option will somehow know
when one tunnel endpoint goes offline and do some sort of failover
between other values provided in other instances of the AFTR option.
Others assumed that if there are multiple options, the client will
somehow do load balancing between the provided tunnel endpoints.
Neither failover nor load balancing was defined for the DS-Lite
architecture, so it caused confusion. It was eventually decided to
allow only one instance of the AFTR option.
17. Option Order
Option order, either the order among many DHCPv6 options or the order
of multiple instances of the same option, SHOULD NOT be significant.
New documents MUST NOT assume any specific option processing order.
As there is no explicit order for multiple instances of the same
option, an option definition SHOULD instead restrict ordering by
using a single option that contains ordered fields.
As [RFC3315] does not impose option order, some implementations use
hash tables to store received options (which is a conformant
behavior). Depending on the hash implementation, the processing
order is almost always different then the order in which the options
appeared in the packet on the wire.
18. Relay Options
In DHCPv4, all relay options are organized as sub-options within the
DHCP Relay Agent Information option [RFC3046]. And, an independent
number space called "DHCP Relay Agent Sub-options" is maintained by
IANA. Different from DHCPv4, in DHCPv6, relay options are defined in
the same way as client/server options, and they also use the same
option number space as client/server options. Future DHCPv6 relay
options MUST be allocated from this single DHCPv6 option number
space.
For example, the Relay-Supplied Options option [RFC6422] may also
contain some DHCPv6 options as permitted, such as the Extensible
Authentication Protocol (EAP) Re-authentication Protocol (ERP) Local
Domain Name DHCPv6 Option [RFC6440].
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RFC 7227 DHCPv6 Option Guidelines May 201419. Clients Request Their Options
The DHCPv6 Option Request Option (OPTION_ORO) [RFC3315] is an option
that serves two purposes -- to inform the server what options the
client supports and what options the client is willing to consume.
For some options, such as the options required for the functioning of
DHCPv6 itself, it doesn't make sense to require that they be
explicitly requested using the Option Request Option. In all other
cases, it is prudent to assume that any new option must be present on
the relevant option request list if the client desires to receive it.
It is tempting to add text that requires the client to include a new
option in the Option Request Option list, similar to this text:
"Clients MUST place the foo option code on the Option Request Option
list, clients MAY include option foo in their packets as hints for
the server as values the desire, and servers MUST include option foo
when the client requests it (and the server has been so configured)".
Such text is discouraged as there are several issues with it. First,
it assumes that client implementation that supports a given option
will always want to use it. This is not true. The second and more
important reason is that such text essentially duplicates the
mechanism already defined in [RFC3315]. It is better to simply refer
to the existing mechanism rather than define it again. See
Section 21 for proposed examples on how to do that.
Creators of DHCPv6 options cannot assume special ordering of options
either as they appear in the Option Request Option or as they appear
within the packet. Although it is reasonable to expect that options
will be processed in the order they appear in ORO, server software is
not required to sort DHCPv6 options into the same order in Reply
messages.
It should also be noted that options values are not required to be
aligned within the DHCP packet; even the option code and option
length may appear on odd-byte boundaries.
20. Transition Technologies
The transition from IPv4 to IPv6 is progressing. Many transition
technologies are proposed to speed it up. As a natural consequence,
there are also DHCP options proposed to provision those proposals.
The inevitable question is whether the required parameters should be
delivered over DHCPv4 or DHCPv6. Authors often don't give much
thought about it and simply pick DHCPv6 without realizing the
consequences. IPv6 is expected to stay with us for many decades, and
so is DHCPv6. There is no mechanism available to deprecate an option
in DHCPv6, so any options defined will stay with us as long as the
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RFC 7227 DHCPv6 Option Guidelines May 2014
DHCPv6 protocol itself lasts. It seems likely that such options
defined to transition from IPv4 will outlive IPv4 by many decades.
From that perspective, it is better to implement provisioning of the
transition technologies in DHCPv4, which will be obsoleted together
with IPv4.
When the network infrastructure becomes IPv6 only, the support for
IPv4-only nodes may still be needed. In such a scenario, a mechanism
for providing IPv4 configuration information over IPv6-only networks
may be needed. See [IPv4-CONFIG] for further details.
21. Recommended Sections in the New Document
There are three major entities in DHCPv6: server, relay agent, and
client. It is very helpful for implementors to include separate
sections that describe operation for those three major entities.
Even when a given entity does not participate, it is useful to have a
very short section stating that it must not send a given option and
must ignore it when received.
There is also a separate entity called the "requestor", which is a
special client-like type that participates in the leasequery protocol
[RFC5007] [RFC5460]. A similar section for the requestor is not
required, unless the new option has anything to do with the requestor
(or it is likely that the reader may think that is has). It should
be noted that while in the majority of deployments the requestor is
co-located with the relay agent, those are two separate entities from
the protocol perspective, and they may be used separately. There are
stand-alone requestor implementations available.
The following sections include proposed text for such sections. That
text is not required to appear, but it is appropriate in most cases.
Additional or modified text specific to a given option is often
required.
Although the requestor is a somewhat uncommon functionality, its
existence should be noted, especially when allowing or disallowing
options to appear in certain messages or to be sent by certain
entities. Additional message types may appear in the future, besides
types defined in [RFC3315]. Therefore, authors are encouraged to
familiarize themselves with a list of currently defined DHCPv6
messages available on the IANA website [IANA].
Typically, new options are requested by clients and assigned by the
server, so there is no specific relay behavior. Nevertheless, it is
good to include a section for relay agent behavior and simply state
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RFC 7227 DHCPv6 Option Guidelines May 2014
that there are no additional requirements for relays. The same
applies for client behavior if the options are to be exchanged
between the relay and server.
Sections that contain option definitions MUST include a formal
verification procedure. Often it is very simple, e.g., an option
that conveys an IPv6 address must be exactly 16-bytes long, but
sometimes the rules are more complex. It is recommended to refer to
existing documents (e.g., Section 8 of RFC 3315 for domain name
encoding) rather than trying to repeat such rules.
21.1. DHCPv6 Client Behavior Text
Clients MAY request option foo, as defined in [RFC3315], Sections
17.1.1, 18.1.1, 18.1.3, 18.1.4, 18.1.5, and 22.7. As a convenience
to the reader, we mention here that the client includes requested
option codes in the Option Request Option.
Optional text (if the client's hints make sense): The client also MAY
include option foo in its Solicit, Request, Renew, Rebind, and
Information-request messages as a hint for the server regarding
preferred option values.
Optional text (if the option contains an FQDN): If the client
requests an option that conveys an FQDN, it is expected that the
contents of that option will be resolved using DNS. Hence, the
following text may be useful: Clients that request option foo SHOULD
also request option OPTION_DNS_SERVERS as specified in [RFC3646].
Clients MUST discard option foo if it is invalid (i.e., it did not
pass the validation steps defined in Section X.Y).
Optional text (if option foo in expected to be exchanged between
relays and servers): Option foo is exchanged between relays and
servers only. Clients are not aware of the usage of option foo.
Clients MUST ignore received option foo.
21.2. DHCPv6 Server Behavior Text
Sections 17.2.2 and 18.2 of [RFC3315] govern server operation in
regards to option assignment. As a convenience to the reader, we
mention here that the server will send option foo only if configured
with specific values for foo and if the client requested it.
Optional text: Option foo is a singleton. Servers MUST NOT send more
than one instance of the foo option.
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RFC 7227 DHCPv6 Option Guidelines May 2014
Optional text (if the server is never supposed to receive option
foo): Servers MUST ignore the incoming foo option.
21.3. DHCPv6 Relay Agent Behavior Text
It's never appropriate for a relay agent to add options to a message
heading toward the client, and relay agents don't actually construct
Relay-reply messages anyway.
Optional text (if the foo option is exchanged between the clients and
server or between requestors and servers): there are no additional
requirements for relays.
Optional text (if relays are expected to insert or consume option
foo): Relay agents MAY include option foo in a Relay-forward message
when forwarding packets from clients to the servers.
22. Should the New Document Update Existing RFCs?
Authors often ask themselves whether their proposal updates existing
RFCs, especially RFC 3315. In April 2013, there were about 80
options defined. Had all documents that defined them also updated
RFC 3315, comprehension of such a document set would be extremely
difficult. It should be noted that "extends" and "updates" are two
very different verbs. If a new document defines a new option that
clients request and servers provide, it merely extends current
standards, so "updates RFC 3315" is not required in the new document
header. On the other hand, if a new document replaces or modifies
existing behavior and includes clarifications or other corrections,
it should be noted that it updates the other document. For example,
[RFC6644] clearly updates [RFC3315] as it replaces existing text with
new text.
If in doubt, authors should try to determine whether an implementor
reading the base RFC alone (without reading the new document) would
be able to properly implement the software. If the base RFC is
sufficient, then the new document probably does not update the base
RFC. On the other hand, if reading your new document is necessary to
properly implement the base RFC, then the new document most likely
updates the base RFC.
23. Security Considerations
DHCPv6 does have an authentication mechanism [RFC3315] that makes it
possible for DHCPv6 software to discriminate between authentic
endpoints and man-in-the-middle. Other authentication mechanisms may
optionally be deployed. Sadly, as of 2014, the authentication in
DHCPv6 is rarely used, and support for it is not common in existing
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RFC 7227 DHCPv6 Option Guidelines May 2014
implementations. Some specific deployment types make it mandatory
(or parts thereof, e.g., DOCSIS3.0-compatible cable modems require
reconfigure-key support), so in certain cases, specific
authentication aspects can be relied upon. That is not true in the
generic case, though.
So, while creating a new option, it is prudent to assume that the
DHCPv6 packet contents are always transmitted in the clear, and
actual production use of the software will probably be vulnerable at
least to man-in-the-middle attacks from within the network, even
where the network itself is protected from external attacks by
firewalls. In particular, some DHCPv6 message exchanges are
transmitted to multicast addresses that are likely broadcast anyway.
If an option is of a specific fixed length, it is useful to remind
the implementor of the option data's full length. This is easily
done by declaring the specific value of the 'length' tag of the
option. This helps to gently remind implementors to validate the
option length before digesting them into likewise fixed-length
regions of memory or stack.
If an option may be of variable size (such as having indeterminate
length fields, such as domain names or text strings), it is advisable
to explicitly remind the implementor to be aware of the potential for
long options. Either define a reasonable upper limit (and suggest
validating it) or explicitly remind the implementor that an option
may be exceptionally long (to be prepared to handle errors rather
than truncate values).
For some option contents, out-of-bound values may be used to breach
security. An IP address field might be made to carry a loopback
address or local multicast address, and depending on the protocol,
this may lead to undesirable results. A domain name field may be
filled with contrived contents that exceed the limitations placed
upon domain name formatting; as this value is possibly delivered to
"internal configuration" records of the system, it may be implicitly
trusted without being validated.
Authors of documents defining new DHCP options are, therefore,
strongly advised to explicitly define validation measures that
recipients of such options are required to do before processing such
options. However, validation measures already defined by RFC 3315 or
other specifications referenced by the new option document are
redundant and can introduce errors, so authors are equally strongly
advised to refer to the base specification for any such validation
language rather than copying it into the new specification.
See also Section 24.
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RFC 7227 DHCPv6 Option Guidelines May 201424. Privacy Considerations
As discussed in Section 23, the DHCPv6 packets are typically
transmitted in the clear, so they are susceptible to eavesdropping.
This should be considered when defining options that may convey
personally identifying information (PII) or any other type of
sensitive data.
If the transmission of sensitive or confidential content is required,
it is still possible to secure communication between relay agents and
servers. Relay agents and servers communicating with relay agents
must support the use of IPsec Encapsulating Security Payload (ESP)
with encryption in transport mode, according to Section 3.1.1 of
[RFC4303] and Section 21.1 of [RFC3315]. Sadly, this requirement is
almost universally ignored in real deployments. Even if the
communication path between the relay agents and server is secured,
the path between the clients and relay agents or server is not.
Unless underlying transmission technology provides a secure
transmission channel, the DHCPv6 options SHOULD NOT include PII or
other sensitive information. If there are special circumstances that
warrant sending such information over unsecured DHCPv6, the dangers
MUST be clearly discussed in the security considerations.
25. Acknowledgements
The authors would like to thank Simon Perreault, Bernie Volz, Ted
Lemon, Bud Millwood, Ralph Droms, Barry Leiba, Benoit Claise, Brian
Haberman, Richard Barnes, Stephen Farrell, and Stewart Bryant for
their comments.
26. References26.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
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